Early Stages of Fatigue Damage of Steels for Fusion Energetics

Hlavním cílem této práce bylo vysvětlit únavové chování ocelí ODS Eurofer, 14Cr ODS feritické oceli vyráběné v CEA (Commissariat a l’énergie atomique, Saclay, France) a 14Cr ODS feritické oceli vyráběné v EPFL (École Polytechnique Fédérale de Laussane, Switzerland). Pokud to bylo možné byly získané výsledky porovnány s ocelí Eurofer 97. Tato práce je součástí široké spolupráce, která je zaměřena na vývoj konstrukčních materiálů pro fuzní reaktory. V průběhu práce byly provedeny následující experimenty: • Standardní únavové zkoušky za pokojové teploty, při 650 a při 750 , byly získány křivky cyklického zpevnění/změkčení, cyklické deformační křivky, Coffinovy–Mansonovy a Wöhlerovi křivky. Hysterezní smyčky byly detailně analyzovány. • Pozorování mikrostruktury výchozího stavu pomocí TEM a porovnání s mikrostrukturou po cyklickém zatížení. • Pozorování povrchového reliéfu, který vznik díky lokalizaci cyklické plastické deformace - byla odhalena místa iniciace únavových trhlin a byl analyzován mechanismus jejich vzniku. • Měření kinetiky únavových trhlin. Oxidická disperse značně zpevňuje materiály, redukuje cyklické změkčení a stabilizuje mikrostrukturu v celém rozsahu teplot. Byly nalezeny velké rozdíly v životnosti jednotlivých ocelí. Ty byly vysvětleny pomocí rozdílných mechanismů iniciace trhlin a jejich různou rychlostí.The main aim of the thesis is to explain fatigue behaviour of the ODS Eurofer steel, 14Cr ODS ferritic steel produced in the CEA (Commissariat a l’énergie atomique, Saclay, France) and 14Cr ODS ferritic steel produced in the EPFL (École Polytechnique Fédérale de Laussane, Switzerland). The results are compared with non–ODS variant of Eurofer steel, if possible. This work is part of a wide collaboration coordinated by the EFDA (European Fusion Development Agreement) on development of structural materials for future fusion reactors. The following experiments were carried out: • Standard fatigue experiments were performed at room temperature, at 650 and at 750 . Cyclic hardening/softening curves, cyclic deformation stress–strain curves, Coffin–Manson curves and Wöhler curves were measured. Detailed analysis of hysteresis loops was performed. • The microstructure of the as–received state was studied by TEM and compared with the microstructure after fatigue loading in order to reveal microstructural changes due to fatigue loading. • Surface roughening due to cyclic plastic localisation was observed. Crack initiation sites were detected and the mechanism of crack initiation was analysed. • Kinetics of small fatigue crack growth was measured. It was found that fine oxide dispersion strengthens studied materials significantly, reduces cyclic softening and stabilises the microstructure in the whole range of testing temperatures. The maximum application temperature is increased. Significant differences in fatigue life of the ODS steels were found and explained by differences in crack initiation mechanisms and small fatigue crack growth rate.

Determination of mechanical properties from microcompression test

This paper describes a microcompression test of thin Al - 1.5 wt. % Cu thin film deposited on Si substrate. Microcompression combines the sample preparation with the use of ion focused beam (FIB) with a compression test carried out using nanoindenter. Cylindrical specimens (pillars) were prepared using FIB. The diameter of pillars was about 1.3 um and their height was about 2 um (equal to the film thickness). Stress-strain curves of the thin film were obtained. The results depend on crystallographic orientation of pillar. The paper is focused to an attempt to determine as precisely as possible Young modulus of the film using experimental data and finite element modelling

Micromechanical testing of Mo‐B‐C layers prepared by magnetron sputtering

Based on attractive mechanical properties of transition metal diborides the group of X2BC ternary compounds (X = Ti, V, Zr, Nb, Mo, Hf, Ta and W) became subject of theoretical calculations [1-3] and lately proved their applicability experimentally [4]. X2BC with X = Mo, Ta and W are the most promising candidates for protection of cutting and forming tools due to their unusually stiffness and moderately ductility. These properties can be understood by considering the electronic structure and particularly the extreme anisotropy (orthorhombic crystal lattice with a=0.309nm, b=1.735nm, c=0.305nm). Please click Additional Files below to see the full abstract

Advantageous Description of Short Fatigue Crack Growth Rates in Austenitic Stainless Steels with Distinct Properties

In this work two approaches to the description of short fatigue crack growth rate under large-scale yielding condition were comprehensively tested: (i) plastic component of the J-integral and (ii) Polak model of crack propagation. The ability to predict residual fatigue life of bodies with short initial cracks was studied for stainless steels Sanicro 25 and 304L. Despite their coarse microstructure and very different cyclic stress-strain response, the employed continuum mechanics models were found to give satisfactory results. Finite element modeling was used to determine the J-integrals and to simulate the evolution of crack front shapes, which corresponded to the real cracks observed on the fracture surfaces of the specimens. Residual fatigue lives estimated by these models were in good agreement with the number of cycles to failure of individual test specimens strained at various total strain amplitudes. Moreover, the crack growth rates of both investigated materials fell onto the same curve that was previously obtained for other steels with different properties. Such a "master curve" was achieved using the plastic part of J-integral and it has the potential of being an advantageous tool to model the fatigue crack propagation under large-scale yielding regime without a need of any additional experimental data

Behavior of W-based materials in hot helium gas

Materials for the plasma facing components of future fusion reactors will be subjected to complex loading and various forms of interaction with low Z species (hydrogen isotopes and helium). The divertor components will be among the most intensely loaded, as they will have to transfer heat loads up to 10–20 MW/m2. Besides the plasma facing surface being irradiated by highly energetic deuterium, tritium and helium particles from the burning plasma, the opposite surface will be exposed to a cooling medium at elevated temperature. Helium- and water-based cooling systems are currently being considered. While tungsten is the prime candidate material for the plasma facing components, in the helium-cooled divertor designs, it is also foreseen as a structural material, together with ferritic–martensitic steels. The behavior of these materials in He atmosphere at elevated temperatures has been little studied thus far, and therefore is the subject of the current work. A number of W-based materials (pure tungsten and some of its alloys) prepared by powder metallurgy techniques was exposed to He atmosphere at 720 C and 500 kPa for 500 h. Morphological surface changes were observed by SEM, chemical and phase composition was analyzed by EDS and XRD, respectively. The internal microstructure was observed by a combination of SEM, FIB and TEM techniques. Mechanical properties were determined by instrumented indentation. Some alloys developed a thin oxide layer, in some cases new morphological features were observed, while some samples remained mostly intact. The observed changes are correlated with specific compositions and microstructures

Measurement of mechanical properties of thin films by nanocompression

Mechanical properties of thin films are not easy to be measured. This is particularly true in the case of plastic properties as the yield point, the work hardening rate or the ultimate stress. Nevertheless, such parameters are needed e.g. in the design of integrated circuits where the thermal stresses may lead to mechanical failure of the component. We applied two modern experimental facilities, the focused ion beam and the nanoindentation for exact measurement of plastic properties of a Al-1.5%Cu thin film prepared by PVD, used for electrical connection of integrated circuits. By FIB milling, cylindrical specimens were prepared. The height of the specimens was equal to the film thickness (2 m) and their diameter was about 1.3 m. These specimens were subjected to the compressive loading using the nanoindenter equipped by a flat punch. Stress-strain curves of the film were obtained rather precisely

Development of Nanofabrication Methodology for Study of Mechanical Properties of Thin Films using Focused Ion Beams

The main goal of this work is to find a methodology of the fabrication of microcompressive specimens (pillars) from thin metallic film prepared by means of PVD. The studied film was prepared by the ON Semiconductor company, Roznov pod Radhostem. Its chemical composition was Al-1.5 wt.% Cu; such films are used for electric connections on integrated circuits. At first, a thin intermediate layer of W-10 wt.% Ti was deposited on the Si single crystalline substrate with the purpose of improving adhesion properties of the studied film. The geometry of the microcompressive specimen should be as close to the cylindrical shape as possible. The height of the cylinder is given by the film thickness, its diameter is approximately 1 m. Such specimens were prepared in Quanta 3D FEG Dual BeamTM facility using focused ion beams technology. Experiments were done at FEI Company in Brno. In total, 39 microcompressive specimens were prepared at various ion milling conditions. The required geometry was finally attained by the optimization of processing parameters, in particular the parallelism of lateral faces was improved, the bottom of the removed zone in the vicinity of the pillar was almost flat and the transition pillar – flat bottom was regular. The prepared pillars are suitable for the microcompression tests; the first of them have been already performed within the cooperation with the Institut of Physics, Academy of Sciences of the Czech Republic, Praha

Determination of mechanical properties of thin films by nanocompresion

Measurements of mechanical properties of objects with micrometric (or even smaller) dimensions is still not a common task. In this paper, the possibility of evaluation basic mechanical properties of a thin film by nanocompression is demonstrated. Cylindrical specimens with the axis normal to the film plane, attached by the bottom to the substrate, are prepared by the focused ion beam technique. Such pillars are deformed by a nanoindenter outfitted by a flat diamond punch. An equivalent of compression curve is obtained. It is possible to measure directly parameters as the yield stress, stress at a chosen strain level or work hardening rate. Finite elements modelling is necessary for the Young modulus evaluation. It is shown that the Young modulus can be evaluated quite precisely, even if the geometry of the pillar is not perfect

Nanocompression of oriented pillars from Al thin film

In this paper, a new method of measurement of mechanical properties of thin films is presented. This method combines specimen preparation by focused ion beam (FIB) and compression test using nanoindentation device. Compression specimens were prepared from thin film, Al-1.5%Cu, which is commonly used in integrated circuit. Cylindrical specimens were prepared by FIB milling. The height of specimens (pillars) was about 2 mircrometers (equal to the film thickness) and their diameter was about 1.3 micrometers. The pillars are single crystalline, therefore the results depend on crystallographic orientation of pillar, which was specified by EBSD (electron backscatter diffraction). Stress-strain curves of the thin film were obtained in two representations

Fatigue properties of Eurofer steel developed for fusion application

In this study fatigue properties of Eurofer 97 steel in room temperatures were measured. Fatigue parameters of the cyclic stress - strain curve and fatigue life curves (Coffin-Manson curve and derived Wöhler curve) were evaluated. Major attention was given to the measurement of kinetics of fatigue crack growth by two different methods. The initiation sites of fatigue cracks and short crack growth were observed and measured on cylindrical specimens with shallow notch. These cracks had a length from 20 micrometers to 1 mm. Kinetics of growth of long cracks with the length 15-30 mm was measured on CT specimens. It was found that results obtained by the both methods are in a good agreement if the J-integral is used. It was possible to determine the threshold values of J-integral and stress intensity factor and to calculate the Paris law parameters